1. Field of the Invention
This invention relates to a design for microphones, and more particularly relates to a wide band microphone with an acoustic equalizer or resonator.
2. Description of the Prior Art
In order to extend the high frequency response of a microphone, it is well known in the art to position a resonator plate in front of the microphone. More particularly, such a plate includes a series of apertures formed through its thickness and situated peripherally about its center. In one known application, the plate may be positioned directed adjacent to the microphone diaphragm, and may include a lip surrounding the periphery of the plate and extending axially in the direction of the diaphragm, which lip abuts against the diaphragm to space it from the plate. The lip, plate and diaphragm thus define a cavity which communicates with the peripherally spaced apertures. This arrangement is commonly known as a Helmholtz resonator, and is frequently used in the industry as a cost saving measure to achieve an increased frequency sensitivity from a relatively low priced microphone.
Some microphones, without being fitted with such a resonator plate, can exhibit a roll-off in their frequency response at about 4 to 5 kHz. The frequency response of such microphones may be extended to as much as 20 kHz by mounting a Helmholtz resonator plate on the microphone, and tuning the resonator plate (by adjusting the thickness of the plate and number and size of the peripherally spaced apertures) to resonate at about 15 kHz.
Such a microphone is usually structured to define a back cavity or chamber of air adjacent to the back face of the diaphragm. Because of size and design constraints, this back chamber of air can resonate at a frequency of about 5 kHz, which causes an undesirable peak in the microphone's frequency response.
If the microphone is used without an equalizer, this undesirable peak in the frequency response is of little or no consequence, being outside the usable 4 kHz bandwidth of the microphone, as mentioned previously. However, when a conventional resonator plate is used in conjunction with the microphone to extend its frequency response, the undesirable peak becomes prevalent in the extended operating frequency range of the microphone, resulting in signal distortion.
A known method of suppressing such peaks is to dampen the signal by covering the peripherally spaced apertures of the resonator plate with a fabric mesh material or by adding similar material in communication with the back cavity or chamber. However, this method has the effect of attenuating the entire frequency response of the microphone, and little may not provide the ability to selectively eliminate an undesirable peak in the frequency response.